Wattmeters have long been employed to monitor the voltage and current of power lines and to indicate the power that is passed by the lines in units of watts or watthours. For example, mechanical watthour meters are now employed by many power companies to measure the amount of power that is used at consumer locations. Such meters are periodically read at the consumer locations by employees of the power company.
It has been suggested that power may be monitored more simply and efficiently by using an electronic wattmeter to continuously transmit measured power to a central monitoring facility. The monitoring facility could then obtain a continuous reading of the power consumption at many remote locations and could use the continuous power measurement to accurately allocate power.
Various wattmeters have been developed to electronically monitor the voltage and current of a power line or lines and to transmit an electrical signal indicating the corresponding power measurement. However, some prior art wattmeters are relatively difficult to install since they can be connected only by breaking or disconnecting a power line. Also, transformers have been used to isolate the voltage and power supply connections of a meter, and such transformers are generally bulky and expensive.
For systems wherein power is both generated and dissipated, it is generally desirable to provide a means for measuring both the magnitude and direction of the flow of power. It is also desirable in some cases to monitor instantaneous power in watts, power usage in watthours and line voltage and current. Moreover, if a wattmeter is to monitor the power conducted over a rigid bus bar, some means must be provided to securely connect the wattmeter to the bar, without disrupting the flow of current along the bar.
Accordingly, it is an object of the invention to provide a relatively simple and reliable apparatus for measuring electrical power and for generating an electrical signal corresponding to the magnitude of the measured power.
A further object of the invention is to provide such an apparatus having a current transducer that may be inductively coupled to a power line without disconnecting or cutting the line.
Another object of the invention is to provide a wattmeter apparatus that operates without a voltage transformer or a power supply transformer and that is optically isolated at its output.
A further object of the invention is to provide a wattmeter having a low power dissipation.
Another object of the invention is to provide a meter that generates a signal having a frequency proportional to measured power in watthours, a signal having a duty cycle proportional to measured power in watts and a polarity indicating the direction of the flow of power and signals having duty cycles proportional to line voltage and current.
A further object of the invention is to provide a wattmeter that may be easily connected to a power bus bar without cutting the bar or otherwise disrupting the flow of current along the bar.